1. Field of the Invention
This invention relates in general to a cordless telephone system. In particular, the invention relates to the implementation of a cordless telephone system using the BLUETOOTH wireless digital communications standard.
2. Background Art
BLUETOOTH is an emerging wireless standard, operating in the 2.4 GHz ISM band world-wide according to existing specifications known in the art and publicly available. Although originally developed for use in the mobile phone industry, widespread adoption within the consumer electronics industry suggests that it is likely to become a popular and commonplace home wireless standard in the coming years. Due to it's predicted penetration into the consumer market, it is of considerable interest to use the BLUETOOTH standard as the bearer for a cordless telephone system.
In it's basic form, a cordless telephone system consists of a basestation that is connected to the Public Switched Telephone Network (“PSTN”) and that communicates wirelessly to a handset. The principal data exchanged between the basestation and handset is voice data (the telephone call), although a small amount of control data and information may also be exchanged. The system can be (and has been) extended to multiple PSTN connections and multiple handsets.
Several characteristics for a wireless communications link are desirable in implementing a cordless telephony platform. Such characteristics include: 1) the provision of at least two full-duplex voice links to support multiple handsets; 2) a low-speed data link in parallel with voice links for system communication such as caller ID information; 3) a high-speed data link to provide for transmission of call setup information with minimal delay; 4) robust error detection and/or error correction on voice and data links to ensure high-quality and uninterrupted communications; and 5) implementation of one or more high-speed data links in parallel with the voice link(s).
However, the transmission formats defined by the BLUETOOTH specification do not provide a combination of these characteristics that is typically desirable for cordless telephony. Three Synchronous Connection-Oriented (“SCO”) packet types are defined in the BLUETOOTH standard specifically for voice transmission (namely, HV1, HV2 and HV3). A SCO link is a point-to-point, circuit-switched connection between a master and a slave. The BLUETOOTH SCO links employ varying error correction schemes where Forward Error Correction (“FEC”) may or may not be available to protect the transmitted data. HV1 uses ⅓ rate FEC and allows 10 bytes of data; HV2 uses ⅔ rate FEC and allows 20 bytes of data; and HV3 uses no FEC and allows 30 bytes of data. The packet header is always protected by ⅓ rate FEC.
The BLUETOOTH HV1 format occupies the complete channel bandwidth for a single link, and is thus unable to support multiple handsets or a parallel data channel. In contrast, the HV3 link occupies a reduced bandwidth but does not contain error detection which is required for implementation of a high-quality voice link. The HV2 format allows two full-duplex voice links but lacks the desired parallel data capability. Such parallel data capabilities may be a primary motivation for moving to a BLUETOOTH wireless bearer. Furthermore, even BLUETOOTH's voice links which provide error correction do not provide error detection. Therefore, packets received with errors that are not properly corrected cannot be readily muted, potentially exposing the data recipient to unpleasant audible noise.
Thus, the voice formats defined by the BLUETOOTH standard are generally unsatisfactory for many advanced cordless telephone applications. However, if the limitations of the formats could be overcome, it would be desirable to implement a BLUETOOTH-based cordless telephone system so that the base unit can also serve as a BLUETOOTH gateway, thereby enabling advanced communications with a variety of electronic devices with minimal added expense.